Bonding using induction heating

ABSTRACT

The processing time and energy efficiency of metal or composite bonding operations is improved by heating the lay-up preform of the structure in an induction heating press where energy goes primarily to heating the workpieces rather than to heating the tooling or being lost to the environment. The bonding process is especially beneficial for joining thermoplastic composites, but it also can be used with metals or thermosets. Preferential heating at the bondline is achieved by using shims to define the bondline and to define bead regions that have relief from contact with the susceptor envelope because of the thickness of the shims in the bondline regions.

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part application based uponU.S. patent application Ser. No. 08/169,655, filed Dec. 16, 1993, nowU.S. Pat. No. 5,530,227 (which was a continuation-in-part application ofU.S. patent application Ser. No. 07/777,739, filed Oct. 15, 1991) nowU.S. Pat. No. 5,410,132. The present application also is acontinuation-in-part application based upon U.S. patent application08/092,050, filed Jul. 15, 1993, now U.S. Pat. No. 5,410,133 and adivisional of U.S. patent application Ser. No. 07/681,004, filed Apr. 5,1991, now U.S. Pat. No. 5,229,562. Finally, the present application isalso a continuation-in-part application based upon U.S. patentapplication Ser. No. 08/151,433, filed Nov. 12, 1993 now U.S. Pat. No.5,420,400. We incorporate these applications and patent by reference.

TECHNICAL FIELD

The present invention is a method for bonding metal or compositestructures, including both thermosets and thermoplastics, usinginduction heating to heat the workpieces efficiently and rapidly and,thereby, to reduce the bonding operations time and cost.

BACKGROUND ART

Fiber-reinforced organic resin matrix composites have a highstrength-to-weight ratio or high stiffness-to-weight ratio and desirablefatigue characteristics that make them increasingly popular as areplacement for metal in aerospace applications where weight, strength,or fatigue is critical. Composites today, however, are expensive. Thereis a need for improved manufacturing processes to reduce touch labor andthe forming time.

Prepregs combine continuous, woven, or chopped fibers with an uncuredmatrix resin, and usually are fiber sheets with a thin film of thematrix. Sheets of prepreg generally are placed (laid-up) directly upon atool or die having a forming surface contoured to the desired shape ofthe completed part or are laid-up in a flat sheet which is then drapedand formed over the tool or die to the contour of the tool. Lay up canbe by hand or with specialized tow or tape placement equipment. Then theresin in the prepreg lay up is consolidated (i.e., cured) in a vacuumbag process in an autoclave (i.e., a pressure oven) to complete thepart.

The tools or dies for metal or composite processing typically are formedto close dimensional tolerances. They are massive, must be heated alongwith the workpiece, and must be cooled prior to removing the completedpart. The delay caused to heat and to cool the mass of the tools addssubstantially to the overall time necessary to fabricate each part.These delays are especially significant when the manufacturing run islow rate where the dies need to be changed after producing only a fewparts of each kind.

In hot press forming, the prepreg is laid-up, bagged (if necessary), andplaced between matched metal tools that include forming surfaces thatdefine the internal, external, or both mold lines of the completed part.The tools and composite preform are placed within a press and then thetools, press, and preform are heated. By "preform" we mean the prepreglay up.

The tooling in autoclave or hot press fabrication is a significant heatsink that consumes substantial energy. Furthermore, the tooling takessignificant time to heat the composite material to its consolidationtemperature and, after curing the composite, to cool to a temperature atwhich it is safe to remove the finished composite part.

As described in U.S. Pat. No. 4,657,717, a flat, composite prepreg panelwas sandwiched between two metal sheets made from a superplasticallyformable alloy, and formed against a die having a surface preciselycontoured to the final shape of the part.

Attempts have been made to reduce composite fabrication times byactively cooling the tools after forming the composite part. Theseattempts have shortened the time necessary to produce a composite part,but the time for and cost of heating and cooling remain significantcontributors to overall fabrication costs. Designing and making toolswith active cooling increases their cost.

Boeing described a process for organic matrix forming and consolidationusing induction heating in U.S. patent application Ser. No. 08/169,655.There, prepregs were laid up in a flat sheet and were sandwiched betweenaluminum susceptor facesheets. To ensure an inert atmosphere around thecomposite during curing and to permit withdrawing volatiles andoutgassing from around the composite during the consolidation, we weldedthe facesheets around their periphery. Such welding unduly impacts thepreparation time and the cost for part fabrication. It also ruined thefacesheets (i.e., prohibited their reuse). The present invention is atechnique that readily and reliably seals the facesheets without theneed for welding and permits reuse of the facesheets in certaincircumstances. Our "bag-and-seal" technique applies to both resincomposite and metal processing, especially in our induction heatingworkcell.

For purposes of this description, we use "consolidation" to meanpressure compacting and curing of an organic matrix resin throughthermally activated chemical reactions to produce a stable composite. By"forming," we mean shaping the composite or metal and retort in itsplastic state. "Forming" may entail superplastic forming, drawing, orsome other shaping operation, as those skilled in the art willunderstand.

Airplanes are commonly made from metal or composite with prefabricatedparts assembled and fastened or riveted together. The labor cost forfastening is a significant cost element and the fasteners add weightthat limits overall performance and capability or adversely impactsoperational costs. For military aircraft, weight translates topayload/range which is critical with modern technology with a smalladvantage can mean the difference between success and failure. Forcommercial aircraft, while weight does not translate to survivability,it is still a significant factor because the capital cost plus theoperating cost are the key elements of the airlines expense.Technologies to reduce cost and weight, accordingly, are dear in theaircraft industry. The present invention is a bonding operation usingBoeing's induction heating workcell which promises significant cost andweight savings for the manufacture of aerospace parts and assemblies.

While graphite or boron fibers can be heated directly by induction, mostorganic matrix composites require a susceptor in or adjacent to thecomposite material preform to achieve the necessary heating forconsolidation or forming. A retort of sealed susceptor sheets is alsodesirable for metal workpieces to control the atmosphere around theworkpiece and to achieve uniform heating. The susceptor is heatedinductively and transfers its heat principally through conduction to thepreform or workpiece that is sealed within the susceptor retort. Whilethe metals in the workpiece may themselves be susceptible to inductionheating, the metal workpiece needs to be shielded in an inert atmosphereduring high temperature processing to avoid oxidation of the metal, sowe usually enclose the workpiece (one or more metal sheets) in a metalretort when using our ceramic tooling induction heating press. Enclosedin the metal retort, the workpiece does not experience the oscillatingmagnetic field which instead is absorbed in the retort sheets. Heatingis by conduction from the retort to the workpiece.

Induction focuses heating on the retort (and workpiece) and eliminateswasteful, inefficient heat sinks. Because the ceramic tools in ourinduction heating workcell do not heat to as high a temperature as themetal tooling of conventional, prior art presses, problems caused bydifferent coefficients of thermal expansion between the tools and theworkpiece are reduced. Furthermore, we are energy efficient becausesignificantly higher percentages of our input energy goes to heating theworkpiece than occurs with conventional presses. Our reduced thermalmass and ability to focus the heating energy permits us to change theoperating temperature rapidly which improves the products we produce.Finally, our shop environment is not heated as significantly from theradiation of the large thermal mass of a conventional press.

In induction heating for consolidating and/or forming organic matrixcomposite materials, we place a thermoplastic organic matrix compositepreform of PEEK or ULTEM, for example, adjacent a metal susceptor. Thesethermoplastics have a low concentration of volatile solvents and areeasy to use. The susceptor facesheets of the retort are inductivelyheated to heat the preform. We apply consolidation and forming pressureto consolidate and, if applicable, to form the preform at its curingtemperature. Generally, we enclose the preform between two susceptorsheets that are sealed to form a pressure zone. We call this assembly a"retort." We evacuate the pressure zone in the retort in a manneranalogous to conventional vacuum bag processes for resin consolidation,or, for resins with low volatiles resins, like ULTEM, we can pressurizethis zone to enhance consolidation. The retort is placed in an inductionheating press on the forming surfaces of dies having the desired shapeof the molded composite part. After the retort (and preform) areinductively heated to the desired elevated temperature, we applydifferential pressure (while maintaining the vacuum in the pressure zonearound the preform) across the retort which functions as a diaphragm inthe press to form the preform against the die into the desired shape ofthe completed composite panel.

The retort often includes three susceptor sheets sealed around theirperiphery to define two pressure zones. The first pressure zonesurrounds the composite panel/preform or metal workpiece and isevacuated and maintained under vacuum. The second pressure zone ispressurized (i.e., flooded with gas) at the appropriate to help form thecomposite panel or workpiece. The shared wall of the three layersandwich that defines the two pressure zones acts as a diaphragm in thissituation.

In preparing the retort, as described in U.S. patent application Ser.No. 08/341,779, we often use temporary seals to hold the sheets untilthe sheets are clamped into the press, and we prefer a "C" spring clamp.The clamp sandwiches the outer susceptor sheets of the retort andprovides a compressive force to hold the retort together temporarily,pressing the sheets against an "O" ring gasket. Such a gasket seatsbetween susceptor sheets in a machined or formed groove or crimp aroundthe periphery of adjacent susceptors. For processing below about 600° F.(315° C.), the gasket is generally silicone rubber. Between about 600°F. (315° C.) and 1300° F. (705° C.), the gasket is copper; above about1300° F. (705° C.), the gasket is stainless steel. The gasket andsusceptor sheets abut and form a gas seal via the compressive force ofthe die set. The "C" clamp permits handling of the retort in and out ofthe die set. The "C" clamp also provides a current path from the topsheet to the bottom sheet (when the gasket is rubber or stainlesssteel). The "C" clamp can be omitted when we use a copper gasket, buthandling the susceptor sheets is more difficult. The "C" clamp jumper isonly required for electrical continuity when the gasket is not anelectrical conductor and, then, only on the edges of the retorttransverse to the induction coils since the coils induce eddy currentsin the susceptor that flow parallel to the coils. We can perform a widerange of manufacturing operations in our induction heating press. Theseoperations have optimum operating temperatures ranging from about 350°F. (175° C.) to about 1950° F. (1068° C.). For each operation, weusually need to hold the temperature relatively constant for severalminutes to several hours while we complete the operations. While we canachieve temperature control by controlling the input power fed to theinduction coil, we have discovered a better and simpler way thatcapitalizes on the Curie temperature. By judicious selection of themetal or alloy in the retort's susceptor facesheets, we can avoidexcessive heating irrespective of the input power. With improved controland improved temperature uniformity in the workpiece, we produce betterproducts.

As described to some degree in U.S. Pat. 4,622,445 and U.S. Pat. No.5,410,132, we discovered an improvement for an SPF process coupling theuse of ceramic dies with inductive heating. With our inductively heatedSPF press or workcell, we can heat preferentially the sheet metalworkpiece with induction heating without heating the platens or diessignificantly and can use the ceramic dies as an insulator to hold theinduced heat in the part. We can stop the heating at any time and cancool the part relatively quickly even before removing it from the die.We do not waste the energy otherwise required to heat the large thermalmass of the platens and dies. We do not force the press operators towork around the hot dies and platens. With our inductive heatingworkcell, we also save time and energy when changing dies to set up tomanufacture different parts because the dies and platen aresignificantly cooler than those in a conventional SPF press. We shortenthe operation to change dies by several hours. Therefore, the inductionheating process is an agile work tool for rapid prototyping or low rateproduction with improved efficiency and versatility.

U.S. Pat. Nos. 3,920,175 and 3,927,817 describe typical combined cyclesfor SPF forming and diffusion bonding. Diffusion bonding is anotoriously difficult and temperamental process that has forced many SPFfabricators away from multisheet manufacturing or to "clean room"production facilities and other processing tricks to eliminate thepossibility of oxidation in the bond. Oxides foul the integrity of thebond. In addition, diffusion bonds are plagued with microvoids which aredifficult to detect nondestructively, but, if present, significantlydiminish the structural performance of the joint. Diffusion bonding alsois a time consuming process. The part typically must be held at elevatedtemperature and elevated pressure (about 400 psi) for several hours. Forexample, in U.S. Pat. No. 3,920,175, the diffusion bonding operationtakes five hours at 1650° F. (900°C.), making the forming/bondingoperation six hours. In U.S. Pat. No. 3,927,817, diffusion bondingoccurs prior to forming, still requires four to five hours, and forces asix hour bonding/forming cycle at 1650° F. (900° C.) for the entireperiod. Typically a hot press diffusion bonding process for commontitanium alloys used in aerospace applications will require over eighthours at 2500 psi and 800° C. (1472° F.), about six hours at 400 psi and900° C. (1650° F.), or about two hours at 250-300 psi and 950° C.(17420° F.). Producing this heat and pressure for this length of time isexpensive.

The present invention is a timesaving process that promises higherquality parts at lower production costs with significant energy savingsin shorter production times. The troubles of diffusion bonding areeliminated by replacing diffusion bonding with brazing so that a muchmore efficient manufacturing cycle is possible. Manufacturers havegreater assurance in the integrity of the brazed bond. To achieve asatisfactory brazed bond quickly and reliably, we focus the heating onthe part we are forming using an induction heater. We hold the partwithin insulating ceramic dies that are transparent to the time-varyingmagnetic field that our induction heater produces. We significantlyreduce cycle time in manufacturing modern aerospace parts.

Our workcell can function with both metals and composites and hasconsiderable versatility and agility with respect to the manufacturingoperations performed and the operating temperature.

SUMMARY OF THE INVENTION

In a method of the present invention for bonding organic matrixcomposite materials, prefabricated resin composite parts (thermosets orthermoplastics) are assembled to define a bondline (for thermosets) orweld line (for thermoplastics). Along the bondline/weld line, we placean adhesive or a thermoplastic which we melt in the bonding operation tojoin the prefabricated composite parts together. We place this assemblyof the prefabricated composite parts and adhesive/weldment within ametal susceptor retort that we seal in accordance with our earlierinventions. We might also use some or all uncured or partially curedparts. When we intend to heat the adhesive/weldment until it melts inthe bonding regions and do not want to melt the prefabricated compositeparts in regions remote from the bondline/weldline, we place susceptorshims on the assembly to relieve the composite parts from (i. e., liftoff the surface of) the metal susceptor sheets in regions other than thebonding/weld line. We load the retort in the induction heating workcell,energize the coil to heat the susceptor, and, in turn theadhesive/weldment while pressing the assembly (at least in the regionsof the bondline) to form the bond upon cooling, and de-energize the coilto cool the retort with whatever cooling profile we prefer.

Because of our capability with induction heating to rapidly heat andcool, we significantly reduce the processing time over conventionalautoclave bonding operations in fabricating thermoset honeycomb panels.While we can heat to 375° F. (190° C.) in about 15 min. with ourworkcell, an autoclave takes about 180 min.; twelve times as long.Therefore, we can shorten the bonding cycle time from as much as 300 minto about 90 min, a reduction of 70%. Time translates directly into cost.For thermoplastic bonding, the cycle time reduction is from about 240min to 30 min (an 87% reduction).

The retort may include three susceptor sheets sealed around theirperiphery to define two pressure zones. The first pressure zonesurrounds the composite panel/preform or metal workpiece and isevacuated and maintained under vacuum. The second pressure zone ispressurized (i.e., flooded with gas) to help form the composite panel orworkpiece. The shared wall of the three layer sandwich acts as adiaphragm in this situation.

For metal bonding, the assembly is similar. Between the prefabricatedmetal parts, however, we use a braze alloy or an organic matrix resinadhesive. We heat the susceptor much hotter than with resin compositeparts, if we are brazing, and usually apply a higher pressure, but wefocus heating along the bondline with the appropriate placement of theshims. The shims conduct heat rapidly from the susceptor to the part inthe location(s) of the bondline while the relief that the shims providein other locations forces convection and radiation to be the primaryheat transfer mechanism to such regions by using shims, we obtain higherpressure at the bondline than at locations remote from the shims withoutchanging the die contour or configuration.

We can also glue metal parts together with a hot melt resin adhesive. Inthis case, we either heat the part directly with induction or we use asusceptor to heat the confined part or we use a bondline susceptor thatis responsive to induction heating. We apply the adhesive to preparedsurfaces and press the parts together with the dies or with diaphragmpressure from pressure zones in the encasing retort. We can bondvirtually any metals this way rapidly using our relatively inexpensive,flexible (multifunctional) tooling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of our induction heating workcell.

FIG. 2 is a schematic cross-sectional view of the apparatus of FIG. 1.

FIG. 3 is a schematic sectional view of our induction heating workcellshowing a typical detail of the edge sealing for our susceptor retort.

FIG. 4 is another schematic sectional view, similar to FIG. 3, showing acopper gasket which allows us to eliminate the "C" clamp used in FIG. 3.

FIG. 5 is another schematic sectional view showing bonding of a beadedpanel using our induction heating workcell.

FIG. 6 is another schematic sectional view showing bonding of ahoneycomb panel to thermoset skins using our induction heating workcell.

FIG. 7 is yet another schematic sectional view showing adhesive bondingof an airfoil, such as two machined halves of an aluminum missile wing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

1. The Induction Heating Process

In FIG. 1, an induction heating workcell 10 includes tools or dies 20and 22 mounted within an upper 24 and a lower 26 strongback. Thestrongbacks are each threaded onto four threaded column supports orjackscrews 28 or they float free on the columns and are fixed with nuts.We can turn the jackscrews to move one strongback relative to the other.The strongbacks 24 and 26 provide a rigid, flat backing surface for theupper and lower dies 20 and 22 to prevent the dies from bending andcracking during manufacturing operations. Preferably, the strongbackshold the dies to a surface tolerance of ±0.003 inches per square foot ofthe forming surface. Such tolerances are desirable to achieve properpart tolerances. The strongbacks may be steel, aluminum, or any othermaterial capable of handling the loads present during forming orconsolidation, but we prefer materials that are non-magnetic to avoidany distortion to the magnetic field that our induction coils produce.In some circumstances, the dies may be strong enough themselves thatstrongbacks are unnecessary. The strongbacks transfer pressure inputthrough the columns evenly to the dies.

The dies 20 and 22 are usually ceramic and are reinforced with aplurality of fiberglass rods 32 that are held with bolts 74 and thatextend both longitudinally and transversely in a grid through each die.Each die usually is framed with phenolic reinforcement 72 as well tomaintain a compressive load on the die. Each die may be attached to itsstrongback by any suitable fastening device such as bolting or clamping.In the preferred embodiment, both dies are mounted on support plates 76which are held in place on the respective strongbacks through the use ofclamping bars 77. The clamping bars 77 extend around the periphery ofthe support plates 76 and are bolted to the respective strongbacksthrough the use of fasteners (not shown).

The dies should not be susceptible to inductive heating so that heatingis localized in the retort. We prefer a ceramic that has a lowcoefficient of thermal expansion, good thermal shock resistance, andrelatively high compression strength, such as a castable fused silicaceramic.

We embed portions of an induction coil 35 in the dies. In theillustrated embodiment, we use four separate induction segments, but thenumber can vary. Each segment is formed from a straight tubing section36 that extends along the length of each die and a flexible coilconnector 38 that joins the straight tubing sections 36 in the upper die20 to the corresponding straight tubing section in the lower die 22.Connectors 40 located at the ends of the induction coil 35 connect theinduction coil 35 to an external power source or coil driver 50 and to acoolant source.

Cavities 42 and 44 in the respective dies can hold tool inserts 46 and48. The upper tool insert 46 in some applications has a contouredforming surface 58 that has a shape corresponding to the desired shapeof the outer mold line surface of the completed composite. The lowertool insert usually determines the inner mold line. The tool insertsalso should not be susceptible to inductive heating, preferably beingformed of a castable ceramic. In some cases, both the dies and the toolinserts can be made from a matrix resin rather than from a as forming orconsolidating certain organic matrix composites. We prefer cast ceramictooling which provides the greatest flexibility and versatility for theinduction heating workcell. The forming surfaces can be an integral partof the dies.

Each die surrounds and supports the respective tool insert and holds thestraight sections 36 of the induction coil in proper position inrelationship to the tool insert 46 or 48. In the preferred embodiment,the interior 70 of the dies is formed of a castable phenolic or ceramicand the exterior sides from precast composite phenolic resin blocks 72.In some applications, we prefer to reinforce the phenolic or ceramicwith chopped fibers or non-woven or woven reinforcing mats.

FIG. 2 shows a retort 60 between the tool inserts 46 and 48. The retort60 includes an organic matrix composite panel or metal workpiecesandwiched with susceptor facesheets. The retort is heated to theoperating temperature by energizing the coil 35. In the case of acomposite panel, when the panel reaches the bonding temperature wherethe adhesive melts or the theromplastic resin flows, we can apply gaspressure to the outer surfaces of the retort by pressure sources 52 and54 to focus the workpiece in regions remote from the bondline. Pressuresource 52 applies pressure to the upper surface of the retort 60 througha conduit 62 that passes through the upper die 20 and upper tool insert46, while pressure source 54 applies a pressure to the lower surface ofthe retort 60 through a conduit 64 that passes through the lower die 22and lower tool insert 48. The pressure applied to the retort 60 ismaintained until the retort has formed to the contour of the formingsurface 58 and the matrix resin has consolidated. The pressure sources52 and 54 generally apply a differential pressure to the retort 60.

Pin holes (not shown) in the tool inserts vent gas trapped between theretort 60 and the forming surface 58 as the retort deforms. Such pinholes can be coupled to a flow meter to monitor the progress of thedeformation.

When the bonding is complete, the induction coil 35 is de-energized andthe pressure relieved. The tool inserts and dies are separated. Weremove the formed retort 60 from the press and recover the compositepart from between the susceptor facesheets.

An alternating oscillating electrical current in the induction coil 35produces a time varying magnetic field that heats the susceptor sheetsof the retort via eddy current heating. The frequency at which the coildriver 50 drives the coils 35 depends upon the nature of the retort 60.We power the coil with up to about 400 kW at frequencies of betweenabout 3-10 kHz. Current penetration of copper at 3 kHz is approximately0.06 inches (1.5 mm), while penetration at 10 kHz is approximately 0.03inches (0.75 mm).

The shape of the coil has a significant effect upon the magnetic fielduniformity. Field uniformity usually is important because temperatureuniformity induced in the retort is directly related to the uniformityof the magnetic field. Uniform heating insures that different portionsof the workpiece will reach the operating temperature at approximatelythe same time. Solenoid type induction coils like those we illustrateprovide a uniform magnetic field, and are preferred. Greater fielduniformity is produced in a retort that is located symmetrically alongthe centerline of the surrounding coil. Those of ordinary skill canestablish series/parallel induction coil combinations, variable turnspacing, and distances between the part and the induction coil bystandard electrical calculations to achieve the desired heating fromwhatever coil configuration is used.

The tool inserts and dies are usually substantially thermally insulatingand trap and contain heat within the retort. Since the dies and toolinserts are not inductively heated and act as insulators to maintainheat within the retort, the present invention requires far less energyto achieve the desired operating temperature than conventional autoclaveor resistive hot press methods where the metal tooling is a massive heatsink.

Bonding operations using our workcell are faster than prior artoperations because we do not heat the large thermal mass of either thedies or tool inserts. The retort is heated, the tool is not. Thus, thenecessary processing temperature is achieved more rapidly. In addition,the highly conductive materials in the retort provide rapid heattransfer to the workpiece. When the driver 50 is de-energized, the diesand the retort cool rapidly to a temperature at which we can remove theretort from the workcell, saving time and energy over conventionalsystems. Coolant flowing through the coil tubes functions as an activeheat exchanger to transfer heat out of the workpiece, retort, and dies.In addition, the thermal cycle is not as limited by the heating andcooling cycle of the equipment and tools so we can tailor the thermalcycle better to the process for which we are using the induction heatingworkcell.

2. Bonding Thermoplastic Resin Composites

In a bonding or bonding-and-forming process for organic (resin) matrixcomposites, a composite panel assembly is laid-up from individualprefabricated composite sheets. The composite panel assembly includes athermoplastic sheet that functions as a bonding agent between theprefabricated parts. The assembly is placed between a first sheet 100and second sheet 102 of a susceptor (usually aluminum, an aluminumalloy, or a "smart" susceptor) to form a retort. As shown in FIG. 4 & 5,the susceptor sheets are sealed around their periphery with a crimp andseal ring 110 formed into the susceptor sheets. A first pressure zone117 between the susceptors 100 and 102 surrounds the composite panel.

In FIG. 4, the susceptor sheets 100 and 102 contact the dies, withoutleaving additional pressure zones between the outer susceptors and thedies, as we described with reference to FIG. 2. When additional pressurezones are used, these zones receive pressurized forming gas through asystem of conduits fabricated in the dies as described in U.S. Pat. Nos.4,708,008; 5,129,249; or 5,309,747 or 5,419,170, which we incorporate byreference. If the retort incorporates the necessary pressure zones toallow forming, the dies can be porous. Such dies do not need to carry orcontain high pressure gases. They are lighter, are easier to make, andare less expensive.

Forming gas is introduced to pressure zone 118 between the twoprefabricated composite panels 200, such as carbon-fiber reinforced PEEKthermoplastic sheets, through suitable pressure lines to deliverpressurized inert gas which forms the panel in its "bead" regionsimultaneously with bonding in the regions to the immediate left andright of pressure zone 118 and the "bead" region, as shown in FIG. 5.

Where we intend to form bonds between the panels 200 and 202, wesandwich a thermoplastic film 204 (generally ULTEM). Such a film shouldmelt at a lower temperature than the thermoplastic in the panels 200 and202 so that bonding can occur without delaminating the panels.

FIG. 5 shows an optional silicone rubber pressure pads 206 between thelower die and the lower susceptor sheet 102. We use these pads 206 toprovide an even pressure across the entire bondline to correct forimperfections in the retort or dies that world otherwise leave isolatedhigh spots. FIG. 5 also shows shim plates 208 between the lowersusceptor sheet 102 and the panel 202 in the bondline region to providerelief in gap 10 between the lower panel 202 and the lower susceptorsheet in the "bead" zone remote from the bondlines.

While we have described that the part can be formed and bondedsimultaneously, the operations can be done sequentially with formingpreceding bonding. Sequential processing is simpler but slower.

We energize the coils with a time varying electrical field to produce atime varying magnetic field to heat the susceptors inductively to thebonding temperature of the composite panel. Heat is transferred byconduction and/or radiation from the susceptors into the compositepanel, so it, too, reaches bonding temperature. The shims transfer heatfaster to the bondline so the ULTEM film bonding agent melts before theremote "bead" regions heat to their delaminating temperature.

After completing bonding, we de-energize the induction coils and coolthe retort and tool inserts to freeze the bonding agent (e.g., theadhesive, thermoplastic, or braze alloy) in the joint. Then we removethe retort from the tool inserts. Although there is some heat transferbetween the retort and the tool inserts, it is insufficient to heat thetool inserts or dies significantly because the dies are such goodthermal insulators. Therefore, the retort can quickly be pulled from thepress. When the retort cools sufficiently, we remove the edge strips andrecover the completed part. Often the edge strips and the susceptorsheets are reusable.

In one example of composite bonding in accordance with the presentinvention, we bonded two composite panels each comprising 48 layers ofthermoplastic PEEK.IM6 prepreg 3/8 inch thick. Two aluminum sheetshaving a thickness of 1/16 inch were placed around the composite paneland aluminum shims were positioned in the bondline region between thelower susceptor and lower PEEK panel as shown in FIG. 5. We used apressure pad as previously described and placed the retort in the toolinserts and inductively heated the assembly to a temperature of 550° F.in 15 minutes We maintained the retort at 550° F. for two minutes andthen cooled it for 15 minutes. When we reached the bonding temperatureof 550° F., we applied a constant pressure of about 100 psi withinpressure zone 117 until the part was cool. These times and pressures arerepresentative only and would differ depending upon the compositematerial used and the thickness and complexity of the bonded part.

The present invention is applicable to all types of organic matrixcomposites including both thermosetting and the thermoplastic compositessuch as epoxies, bismaleimides, polyimides, PEEK, PEK, PEKK, PES, or thelike. If the materials have high solvent concentrations or the resinsemit volatiles when they cure, we need to "bag" the workpiece to permitegress of these volatiles. Therefore, we prefer using resins with lowvolatiles that are true thermoplastics, like PEEK.

We can bond metals in a comparable process that we will describe ingreater detail in section 4.

3. Bonding (i.e., curing) Thermosetting Resin Composite Honeycomb Panels

FIG. 6 illustrates the bonding of thermoset facesheets 300 and 302 to ahoneycomb core 304 using an adhesive film 306 in a cobond operation. Thethermoset sheets and core are contained within two "6061" aluminum alloysusceptor sheets with silicone or Shinitzu rubber overlay 308 configuredto provide a lay-up of substantially constant thickness despite changesin the part configuration. FIG. 6 shows a three susceptor sheet retortwith two sheets 310 and 312 defining a pressure zone in which we caninject argon gas 314 to increase the bonding pressure. Bonding pressureis transferred through the susceptor 310 to the rubber overlay 308 andinto the upper thermoset sheet 300 when the temperature activatesadhesive, typically around 375° F. for adhesives used with epoxythermosets. The third susceptor sheet 316 underlies the facesheet 302.The bonding time is about 1 hour with heatup and cool-down taking about15 min. each for a total cycle time of 90 mins.

The surface of an aircraft wing skin must be maintained to a closetolerance to achieve an efficient aerodynamic surface. The tolerances ofthe inner mold line surface of the wing skin must also be maintained ata close tolerance at least in a buildup area where the wing skin will bejoined to a spar to ensure that the wing skin and spar can be preciselyjoined. It is not critical, however, to control the inner mold linesurface in areas where the wing skin is not attached to otherstructures. The composite panel has additional plies to define thebuildup areas. The additional reinforce the composite panel in theseareas which is necessary where a spar will be attached, and provide aconvenient way to match the skin and spar to produce the desired outerwing configuration even if the spars are imprecise in their dimensions.We can fabricate built up areas at the faying surfaces to provide theprecision fit, in which case we can eliminate shims.

4. Bonding Metals

a. Brazing

For brazing metal we prefer to use copper susceptor sheets about0.032-0.062 inches thick, stainless steel, or a cobalt alloy "smart"susceptors as described in our pending U.S. patent application Ser. No.08/469,604 entitled: "Method for Achieving Thermal Uniformity inInduction Processing of Organic Matrix Composites or Metals," Sometimes,the part itself can be the susceptor. Copper is a good susceptor andallows higher processing temperatures than aluminum or aluminum alloys.Copper also is an excellent susceptor material because it providesexcellent heat transfer. Copper's excellent heat transfer propertieshelp to ensure improved thermal uniformity. When processing metals,especially when using a copper retort at high temperatures, we typicallyuse an inert atmosphere within the interior pressure zones to protectthe copper and the parts being produced (i.e. the workpiece) againstoxidation. That is, the inert gas purge protects the workpiece (and thecopper retort materials) from oxygen damage. The susceptor can be alaminate of aluminum and copper. Aluminum outer sheets efficientlyconvert the magnetic energy to heat. The underlying copper sheetstransfer the heat well from the aluminum skin to the metal or resinworkpiece.

Brazing of metals is essentially the same process as that described forthe bonding of organic matrix composites, so we will not repeat it here.We substitute a braze alloy for the thermoplastic film or thermosetadhesive, and form braze joints between the metal sheets at selectedlocations. We can combine brazing with forming or with other operations.

b. Adhesive bonding

As shown in FIG. 7, the present invention also contemplates adhering twometal parts together using a hot melt, organic matrix resinthermoplastic or thermoset adhesive. In FIG. 7, complementary machinedaluminum missile fin halves 400 and 402 are bonded together with athermoset adhesive 404 in the induction heating workcell. The adhesiveis applied to the prepared surfaces that abut. In the processillustrated, we heat the parts directly with the induction coil 36 thatsurrounds the parts. This adhesive bonding process uses less expensivetooling than alternative processes, accomplishes the bonding in shortthermal cycles, and is faster and cheaper than batch mode autoclavebonding operations. The process is just one more operation that oneworkcell can accomplish, thereby showing the versatility and agility ofthis flexible manufacturing workcell.

While we have described preferred embodiments, those skilled in the artwill readily recognize alterations, variations, and modifications whichmight be made without departing from the inventive concept. Therefore,interpret the claims liberally with the support of the full range ofequivalents known to those of ordinary skill based upon thisdescription. The examples are given to illustrate the invention and notintended to limit it. Accordingly, limit the claims only as necessary inview of the pertinent prior art.

We claim:
 1. A method for bonding together at least two parts usinginduction heating to reduce processing time and energy consumption,comprising the steps of:(a) assembling at least two workpieces into anassembly, the workpieces sandwiching at least one bonding materialselected from the group consisting of temperature-activated adhesives,braze alloys, and bonding agents; (b) placing the assembly within asolenoid induction coil embedded in ceramic dies for providing auniformity of magnetic field in an induction heating press; (c)energizing the induction heating press to heat the bonding materialuntil the bonding material melts while limiting heat loss with theceramic dies; (d) de-energizing the induction heating press causing thebonding material to cool and to bond the workpieces together; (e)recovering the bonded workpieces from the press.
 2. The method of claim1 wherein the workpieces are thermoset resin composites and the bondingmaterial is a heat-activated composition suitable for bonding thethermoset workpieces.
 3. The method of claim 2 further comprisinghoneycomb sandwiched by the thermoset workpieces wherein the bondingmaterial bonds the thermoset workpieces to the honeycomb.
 4. A methodfor bonding together at least two parts using induction heating toreduce processing time and energy consumption, comprising the stepsof:(a) assembling at least two workpieces into an assembly, theworkpieces sandwiching at least one bonding material selected from thegroup consisting of temperature-activated adhesives, braze alloys, andbonding agents; (b) enclosing the assembly in a sealable retort, theretort including at least two susceptor sheets of a metal susceptible toheating inductively; (c) placing the assembly within an induction coilembedded in ceramic in an induction heating press; (d) energizing theinduction heating press to heat the bonding material until the bondingmaterial melts while limiting heat loss with the ceramic dies; (e)de-energizing the induction heating press causing the bonding materialto cool and to bond the workpieces together; and (f) recovering thebonding workpieces from the press;wherein the energizing step heats thebonding material by heating the retort with subsequent transfer of heatfrom the retort to the bonding material through the workpieces.
 5. Themethod of claim 4 wherein the coil is induction heating press includesceramic dies in which a solenoid coil so that the assembly is positionedwithin the coil for heating and wherein the heating involves flowingalternating current through the coil at a frequency of about 3-10 kHz.6. The method of claim 4 wherein the workpieces are thermoplastic resincomposites and the bonding material is a thermoplastic.
 7. The method ofclaim 4 further comprising the step of placing shims between the retortand the workpieces to define a bondline where the workpieces will bebonded and bead regions where the workpieces do not contact thesusceptor sheets.
 8. The method of claim 7 further comprising the stepof forming at least one workpiece in a bead region.
 9. A method of claim7 wherein each workpiece is sheet metal or fiber reinforced resincomposite.
 10. The method of claim 9 wherein the workpieces arethermoplastic resin matrix composites and the bonding material is athermoplastic film.
 11. The method of claim 9 wherein the workpieces arethermoset resin matrix composites.
 12. The method of claim 9 wherein theworkpieces are metal or metal alloys and the bonding material is a brazealloy.
 13. The method of claim 12 wherein the bonding material is abraze alloy.
 14. The method of claim 12 wherein the bonding material isan organic matrix resin adhesive.
 15. The method of claim 12, whereinthe assembly is an airfoil.